from russian 'yes', used with the well-known "retard entonation", means "yes, what else?" meaning the answer is so obvious that any retard would know… and all the retards speak russian, it implies…
So, it's really a kind of insult to the russians coming back from… daaah! what else? the Cold War days, when they were the Evil Ones.
(At least that's what I… grasped, if you will, the first time I've heard it… but not being english my mother-tongue, I'm probrably wrong)

Situation 1: A word used to veer off of a subject or a response to a question or statement that you don't know how to respond to.

Situation 2: This word could be used to retaliate from a verbal attack in a friendly and joking way.

This was first used as an inside joke when one friends status was in German and the other friend commented "Dah" because he didn't understand the status as a joke. Then, for every other comment he didn't know how to respond to he said "Dah"

Q: "I heard you kissed that ugly person at that party last night."
A: "Uhhhhh Dah?"